Contemporary law enforcement and private security organizations must be able to surreptitiously follow vehicles, aircraft, objects, and people. In the past, small relatively low powered radio transmitters were secreted on the object to be followed. Radio direction finding equipment was then deployed to locate and acquire the object. The transmitting device is generally referred to as a beacon or tag, the process of placing the device on the object is referred to as tagging, and locating and following the tag is referred to as tracking.
Today's typical tag is a relatively low powered very high frequency radio transmitter that is manually activated when placed on the object to be tracked. It continually transmits a code of some sort until either the battery energy is depleted or the tracking operation is completed (U.S. Pat. No. 4,021,807 to Culpepper et al). Some tags can change their transmitted code based on local activity (motion, temperature change, etc.). These tags must have external sensors and specially designed electronic logic to accomplish their tasks. Other more advanced tags are commandable (U.S. Pat. No. 4,596,988 to Wanka). A radio signal from a remote unit can cause the transmissions to be turned on or off, or the transmission pattern can be modified as the situation dictates. This kind of tag needs a radio receiver to acquire the command signals, logic of some type to interpret and translate the signals into instructions, and a transmitter that is capable of carrying out the instructions.
Conventional tags have inherent weaknesses. Since a low power tag transmits over a relatively short distance, the only way to track it is to constantly monitor its transmissions with an appropriate radio receiver. Either full time field staff follow and monitor the tag during the tracking period, or a central office monitors the tag by using a system of dedicated receiving antennas that have been installed throughout the tracking area. Both choices are costly and impractical for a realistic national system. Further, the mission fails if the tag moves outside of the effective range of the tracking radio or of the antenna field.
There have been several creative enhancements to the conventional tag. Transmission characteristics have been modified to lower the probability of electronic detection (spread spectrum, microburst, etc.). Transmitters have been miniaturized for such applications as avian research (Telonics, Inc.). Several manufacturers have integrated position location systems, such as LORAN (U.S. Pat. No. 4,742,357 to Rackley) and GPS, with tags to allow tracking from a centralized location. QUALCOMM, GEOSTAR, and others have integrated position location systems with satellite transmitters. Cellular radiotelephones have been proposed as transmitting and receiving conduits for large, complex vehicle location systems (U.S. Pat. No. 5,218,367). Each of these techniques works in its intended application area. However, none adequately addresses the situation of tracking small, light, valuable objects in an urban environment.
Tags that use dedicated on board position location systems suffer from other problems. Their size, weight, and power consumption levels render them unacceptable for many tagging applications. The combined size and combined weight of the position location device, and the transmitter and receiver (for a commandable tag), could easily be greater than the object to be tracked. This is unfeasible.
Ideally a tag designed specifically for small objects would be installed long before the tracking need arose. It would lie dormant, in a power off state and thus retain sufficient battery energy to carry out its mission when activated. The tag would be commandable, would be tracked remotely, and would require intervention by field personnel only during final acquisition and apprehension. It would also have wide, and preferably, national coverage. There is no currently available tag with these characteristics. The disclosed tagging/tracking system meets these criteria. It solves the problems of size, weight, and energy consumption, and provides urban position location adequate for most law enforcement purposes.